The present invention relates to a facsimile signal coding system which permits the receiving station to freely select the quality of a reproduced image, and more particularly to a facsimile signal coding system of particular utility when employed in conversational image communication or image data base retrieval which combines a facsimile terminal with a display unit.
Conventional facsimile communication is paper-to-paper communication and is usually intended to obtain a hard copy. However, there is a tendency that the demand for image processing will be diversified in the future, but at present, the prior art does not possess functions which cope with such a situation.
It is considered that the diversification of facsimile communication will involve the combined use of a facsimile terminal and a display unit for conversational image communication and video data base retrieval. In such conversational image communication, for realizing a smooth conversation in the case of graphic information having a large amount of data, a progressive coding system which provides a rough display of the entire image on a display in as early a stage as possible and then gradually improves the picture quality is more effective than a conventional image coding system which successively reproduces complete pictures along scanning lines from the top to the bottom of the image.
With the sequential progressive coding system, the receiver can decide, from the rough display, whether the information being transmitted is desired one, and if not, he can stop the subsequent unnecessary data transmission. If the information is desired one, then its image quality is improved until the receiver is satisfied, and if necessary, a hard copy of the picture at that time can also be obtained. Thus, the progressive coding system is a coding system that permits the selection of picture quality, a rapid retrieval and curtailment of communication costs, and hence is suitable for interactive image communication. Especially, the sequential progressive coding system is effective for a graded image since it has a large amount of information.
The quality of the graded image depends upon resolution and the number of gradation levels. Accordingly, one possible method for progressively improving the image quality by the progressive coding system is to gradually enhance resolution, and the other is to gradually increase the number of gradation levels. Conventional progressive coding systems for a graded image employ the method of gradually increasing only the number of gradation levels, the method of gradually improving resolution alone or a method of simultaneously increasing the number of gradation levels and resolution in accordance with predetermined algorithms. None of the prior art progressive coding systems is capable of increasing the number of gradation levels and raising resolution independently of each other.